1. Field of the Invention
This invention relates to a surface acoustic wave (SAW) device which can be used in mobile communication devices, more specifically to a surface acoustic wave device which can be used in a filter to penetrate electric signals having a certain frequency.
2. Description of the Related Art
With the development of mobile communication, demand for acoustic surface wave filters for mobile communication devices with high quality characteristics have been increased. Surface acoustic wave filters have been broadly used as a band pass filter in high frequency circuits of mobile communication devices. Known examples of electrode patterns of surface acoustic wave filters include a resonator type as disclosed in P.111-114 of Proceedings on 1992 Ultrasonic Symposium in 1992, Japanese Patent Application Laid Open No. 19044/1977 and Japanese Patent Application Laid Open No. 183380/1993, and an interdigitated interdigital transducer (IIDT) type as disclosed in P.2-8 of IEEE Transactions on Vehicular Technology Vol. 38 No. 1 in 1989 and Japanese Patent Application Laid Open No. 154917/1983. Such resonator type filters and interdigitated interdigital transducer type filters are used as unbalanced type devices for inputting or outputting unbalanced signals.
In general, forming a balanced circuit as a high frequency circuit is advantageous in various aspects. Since a grounded wiring is unnecessary in principle, a high frequency circuit can be designed more freely to allow downsizing of the device. Further, noise reduction can be achieved and low voltage drive in an active device including an amplifier can be enabled. Consequently, by forming a balanced circuit as a high frequency circuit, downsizing and quality improvement of mobile communication devices including cellular phones can be realized. However, since various devices comprising a high frequency have not been comprised of a balanced circuit yet, unbalanced type devices are mainly used in a high frequency circuit with respect to the matching with peripheral circuits.
On the other hand, forming a balanced circuit in a surface acoustic wave filter is feasible in principle, and thus practical application of surface acoustic wave filters in mobile communication devices are expected as disclosed in Proceedings on 1994 MTT-Symposium (1994) WD4A-2.
However, when both a balanced type device and an unbalanced type device are used, a balanced-to-unbalanced transformer circuit to transform a balanced signal to an unbalanced signal is necessary. As a consequence, although characteristics of various high frequency parts are improved, the increase of the number of parts prohibits the downsizing and production cost reduction of the device.
The object of the present invention is to provide a surface acoustic wave device which does not require a balanced-to-unbalanced transformer circuit whether peripheral circuits of the balanced type surface acoustic wave filter comprise a balanced type device or an unbalanced type device.
In order to achieve the above-mentioned object, a first aspect of the surface acoustic wave devices of the present invention comprises a transmitting interdigital transducer with one terminal grounded, a receiving interdigital transducer to receive and transform the surface acoustic waves generated by the transmitting interdigital transducer into electric signals, and a balanced type surface acoustic wave filter connected to the receiving interdigital transducer.
A second aspect of the surface acoustic wave devices of the present invention comprises a balanced type surface acoustic wave filter, a transmitting interdigital transducer connected to the balanced type surface acoustic wave filter, and a receiving interdigital transducer with one terminal grounded which is to receive and transform the surface acoustic waves generated by the transmitting interdigital transducer into electric signals.
A third aspect of the surface acoustic wave devices of the present invention comprises a first transmitting interdigital transducer with one terminal grounded, a first receiving interdigital transducer to receive and transform the surface acoustic waves generated by the first transmitting interdigital transducer to electric signals, a balanced type surface acoustic wave filter connected to the first receiving interdigital transducer, a second transmitting interdigital transducer connected to the balanced type surface acoustic wave filter, and a second receiving interdigital transducer with one terminal grounded which is to receive and transform the surface acoustic waves generated by the second transmitting interdigital transducer into electric signals.
In the above mentioned first to third aspects of the present invention, it is preferable to locate a plurality of transmitting interdigital transducers and a plurality of receiving interdigital transducers alternately to the propagation direction of surface acoustic waves.
In the above mentioned first to third aspects of the present invention, it is further preferable to provide reflectors at outer ends of the transmitting interdigital transducers and receiving interdigital transducers.
A fourth aspect of the surface acoustic wave devices of the present invention comprises a transmitting interdigital transducer with one terminal grounded, first and second receiving interdigital transducers, each with one terminal grounded and located at both sides of the transmitting interdigital transducer to receive surface acoustic waves generated by the transmitting interdigital transducer as surface acoustic waves having different phases and transform them to electric signals, and a balanced type surface acoustic wave filter connected to the first and second receiving interdigital transducers.
A fifth aspect of the surface acoustic wave devices of the present invention comprises a balanced type surface acoustic wave filter, first and second transmitting interdigital transducers with one terminal grounded and connected to the balanced type surface acoustic wave filter, and a receiving interdigital transducer with one terminal grounded and located to receive surface acoustic waves generated by the first and second transmitting interdigital transducers and transform them to electric signals.
A sixth aspect of surface acoustic wave devices of the present invention comprises a first transmitting interdigital transducer with one terminal grounded, first and second receiving interdigital transducers, each with one terminal grounded and located at both sides of the first transmitting interdigital transducer to receive surface acoustic waves generated by the first transmitting interdigital transducer as surface acoustic waves having different phases and transform them to electric signals, a balanced type surface acoustic wave filter connected to the first and second receiving interdigital transducers, second and third transmitting interdigital transducers, each with one terminal grounded and connected to the balanced type surface acoustic wave filter, and a third receiving interdigital transducer with one terminal grounded, and located to receive surface acoustic waves generated by the second and third transmitting interdigital transducers and transform them to electric signals.
In the above mentioned fourth to sixth aspects of the present invention, it is preferable to locate a plurality of transmitting interdigital transducers and a plurality of receiving interdigital transducers alternately to the propagation direction of surface acoustic waves.
In the above mentioned fourth to sixth aspects of the present invention, it is further preferable to provide reflectors at the outer ends of the transmitting interdigital transducers and receiving interdigital transducers.
In the above mentioned first to sixth aspects of the present invention, it is further preferable that the balanced type surface acoustic wave filter comprises four surface acoustic wave resonators connected in a lattice. In this case, it is preferable that a plurality of balanced type surface acoustic wave filters are connected and used.
In the above mentioned first to sixth aspects of the present invention, it is preferable that the frequency at which the insertion loss of frequency characteristics yielded by the transmitting interdigital transducer and the receiving interdigital transducer is at a minimum is approximately the same as the center frequency of the balanced type surface acoustic wave filter.
In the above mentioned first, third, fourth and sixth aspects of the present invention, it is preferable to provide an impedance matching circuit between the output terminals of the receiving interdigital transducer and the input terminals of the balanced type surface acoustic wave filter.
In the above mentioned second, third, fifth and sixth aspects of the present invention, it is preferable to provide an impedance matching circuit between the output terminals of the balanced type surface acoustic wave filter and the input terminals of the transmitting interdigital transducer.
In the above mentioned first to sixth aspects of the present invention, it is preferable that the physical properties of the substrate on which the transmitting interdigital transducer and the receiving interdigital transducer are formed differ from the physical properties of the substrate on which the balanced type surface acoustic wave filter is formed. In this case, it is preferable that the electro-mechanical coupling coefficient of the substrate on which the transmitting interdigital transducer and the receiving interdigital transducer are formed is greater than the electro-mechanical coupling coefficient of the substrate on which the balanced type surface acoustic wave filter is formed. In this case, it is preferable that the frequency temperature coefficient of the substrate on which the transmitting interdigital transducer and the receiving interdigital transducer are formed and the frequency temperature coefficient of the substrate on which the balanced type surface acoustic wave filter is formed are approximately the same. In this case, it is preferable that the substrate on which the transmitting interdigital transducer and the receiving interdigital tranducer are formed and the substrate on which the balanced type surface acoustic wave filter is formed are located in an airtight container. In this case, it is more preferable that the impedance matching circuit located between the output terminals of the receiving interdigital transducer and the input terminals of the balanced type surface acoustic wave filter is located in an airtight container, and further the airtight container comprises a multi-layered dielectric substrate. In this case, it is more preferable that the impedance matching circuit between the output terminals of the balanced type surface acoustic wave filter and the input terminals of the transmitting interdigital transducer is located in an airtight container, and further the airtight container comprises a multi-layered dielectric substrate.
In the above mentioned first to sixth aspects of the present invention, it is preferable that the transmitting interdigital transducer, the receiving interdigital transducer and the balanced type surface acoustic wave filter are formed on the same substrate. In this case, it is preferable that the film thickness of the electrode comprising the transmitting interdigital transducer and the receiving interdigital transducer is thicker than the film thickness of the electrode comprising the balanced type surface acoustic wave filter. In this case, it is further preferable that the impedance matching circuit located between the output terminals of the receiving interdigital transducer and the input terminals of the balanced type surface acoustic wave filter is located in an airtight container, and further the airtight container comprises a multi-layered dielectric substrate. In this case, it is further preferable that the impedance matching circuit located between the output terminals of the balanced type surface acoustic wave filter and the input terminals of the transmitting interdigital transducer is located in an airtight container, and further the airtight container comprises a multi-layered dielectric substrate.